The cover of the November, 1948 issue shows the "Tadpole", a model flying boat that is powered by Carbon Dioxide (CO2). Plans for the model were included in the issue.

Plans for CO2-Powered Flying Boat Tadpole
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In case you were wondering, here is an animation that shows how the CO2 engine works. These engines have become quite popular, and multi-cylinder designs are available. With new materials, the weight of the CO2 cartridge can be reduced allowing for longer flights.

Here is some video of a modern carbon dioxide motor being fueled and tested.

Here is some video of a 21st century flying boat that is very similar to the "Tadpole"